1. Field of Invention
This invention pertains to methods and apparatus for performing RTD and thermocouple cross-calibration in nuclear power plants. More particularly, this invention pertains to using data acquired by a plant monitoring system to calibrate hot leg and cold leg temperature instrumentation in a pressurized water reactor.
2. Description of the Related Art
Pressurized water reactors (PWRs) produce heat through a nuclear reaction in a reactor vessel. The heat is extracted from the reactor vessel by pumping water from the reactor vessel to one or more steam generators. The steam generator is a heat exchanger that extracts the heat from the reactor water into steam that drives a turbine. The piping carrying the heated water from the reactor vessel is called the hot leg, and the piping carrying the cooled water back into the reactor vessel is called the cold leg.
In order to maintain control of the reactor system, the temperature of the reactor water in the hot leg and the cold leg is monitored during reactor start up, shut down, and normal operation. It is common practice to use redundant resistance temperature devices (RTDs) in this application.
Additionally, the temperature of the heated water as it leaves the reactor core is measured by core-exit thermocouples (CETs). A core-exit thermocouple system allows the continuous, on-line monitoring of the coolant temperature at the exit of about one fourth of the fuel assemblies. In present practice, these core-exit thermocouples are installed at or just above the outlet nozzles of a fraction of the fuel assemblies in most commercial pressurized water nuclear power reactors. Typical reactor cores generally consist of from approximately one hundred to more than two hundred fuel assemblies and the core-exit thermocouples are usually located at approximately one out of four fuel assemblies.
Typically, an on-line plant process control computer periodically samples the hot and cold leg RTD resistance and the core-exit thermocouple voltages. These values are converted to convenient engineering units, for example, degrees Fahrenheit or degrees Celsius.
The temperatures measured by the RTDs and CETs are used by the plant operators for process control and to assess the safety of the plant as well as the overall efficiency of power generation. Because the measurements of the RTDs and CETs play a critical role in the evaluation of the plant's operating status, the calibration of the RTDs and CETs are normally evaluated at least once every refueling cycle. Because of plant operating constraints, calibration typically occurs during plant shutdown periods, such as when the reactor core is being refueled, which can occur on an 18-month cycle. Each RTD and CET instrument must meet specific requirements for the plant to continue to produce power according to its design specifications.
In a typical nuclear power plant design, redundant RTDs and CETs are placed in the plant's fluid loops to minimize the probability of failure of any one RTD or CET having a serious effect on the operator's ability to safely and efficiently operate the plant. This redundancy of temperature measurements is the basis for a method of evaluating the calibration of RTDs and CETs called ‘cross calibration’. In cross calibration, redundant temperature measurements are averaged to produce an estimate of the true process temperature. The measurements of each individual RTD and CET are then compared with the process estimate. If the deviations from the process estimate of an RTD or CET is within acceptable limits, the sensor is considered in calibration. However, if the deviation exceeds the acceptance limits, the sensor is considered out of calibration and its use for plant operation must be evaluated.
FIG. 1 illustrates two prior art methods of performing cross calibrations, along with a third method in accordance with the present invention. The plant process 102 is monitored by plant instruments 104, such as RTDs and CETs. The first prior art method of performing cross calibrations is to collect manual measurements 106 of the instruments, and then perform manual calculations 108 to produce the cross calibration results 110. A second prior art method of performing cross calibrations is to use a dedicated data acquisition system 112 to collect the data and produce the results 114. The typical process for performing the cross-calibration occurs when the plant is shutting down for a refueling outage or starting up after an outage when the fluid temperatures go through ranges allowing measurements over the sensor range. The procedure for obtaining the sensor measurements involves physically disconnecting the RTDs or CETs from the plant indications and making measurements using a multimeter or dedicated data acquisition systems. The measurement data is then presented to the plant engineers and used to assess the sensor calibrations with the help of software or manual calculations. After the cross calibration analysis is performed the sensors are connected to the instrumentation to provide indication to the operators.
These prior art methods have the disadvantage of removing the instruments from service for the period measurements are taken, resulting in less information being provided to the plant operators. Additionally, the prior art methods require time and manpower to perform the cross calibrations. Attaching the equipment for the manual measurements 106 or the dedicated data acquisition system 112 requires a trained technician to make the connections and take the actual measurements.